all: refactor extensions, add proto.GetExtension etc.

Change protoiface.ExtensionDescV1 to implement protoreflect.ExtensionType.

ExtensionDescV1's Name field conflicts with the Descriptor Name method,
so change the protoreflect.{Message,Enum,Extension}Type types to no
longer implement the corresponding Descriptor interface. This also leads
to a clearer distinction between the two types.

Introduce a protoreflect.ExtensionTypeDescriptor type which bridges
between ExtensionType and ExtensionDescriptor.

Add extension accessor functions to the proto package:
proto.{Has,Clear,Get,Set}Extension. These functions take a
protoreflect.ExtensionType parameter, which allows writing the
same function call using either the old or new API:

  proto.GetExtension(message, somepb.E_ExtensionFoo)

Fixes golang/protobuf#908

Change-Id: Ibc65d12a46666297849114fd3aefbc4a597d9f08
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/189199
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
diff --git a/testing/prototest/prototest.go b/testing/prototest/prototest.go
index fded0ff..9ff58f9 100644
--- a/testing/prototest/prototest.go
+++ b/testing/prototest/prototest.go
@@ -47,7 +47,7 @@
 		})
 	}
 	for _, xt := range opts.ExtensionTypes {
-		testField(t, m1, xt)
+		testField(t, m1, xt.Descriptor())
 	}
 	for i := 0; i < md.Oneofs().Len(); i++ {
 		testOneof(t, m1, md.Oneofs().Get(i))
@@ -57,12 +57,12 @@
 	// Test round-trip marshal/unmarshal.
 	m2 := m.ProtoReflect().New().Interface()
 	populateMessage(m2.ProtoReflect(), 1, nil)
-	b, err := proto.Marshal(m2)
+	b, err := (proto.MarshalOptions{AllowPartial: true}).Marshal(m2)
 	if err != nil {
 		t.Errorf("Marshal() = %v, want nil\n%v", err, marshalText(m2))
 	}
 	m3 := m.ProtoReflect().New().Interface()
-	if err := proto.Unmarshal(b, m3); err != nil {
+	if err := (proto.UnmarshalOptions{AllowPartial: true}).Unmarshal(b, m3); err != nil {
 		t.Errorf("Unmarshal() = %v, want nil\n%v", err, marshalText(m2))
 	}
 	if !proto.Equal(m2, m3) {